In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, mobile terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a base station, a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband based on WCDMA (Wideband Code Division Multiple Access) and/or HSPA (High Speed Packet Access). Moreover, fuelled by introduction of new devices designed for data applications, end user performance requirements are steadily increasing. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by HSPA networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently may be desired.
Techniques to improve downlink performance may include 4-branch MIMO, multiflow communication, multi carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for HSDPA may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. Currently, heterogeneous networks are being developed for 3GPP as discussed, for example, in: RP-121436, Study on UMTS Heterogeneous Networks, TSG RAN Meeting #57, Chicago, USA, 4th-7th September 2012; R1-124512, Initial considerations on Heterogeneous Networks for UMTS, Ericsson, ST-Ericsson, 3GOO TSG RAN WG1 Meeting #70bis, San Diego, Calif., USA, 8th-12th Oct. 2012; and R1-124513, Heterogeneous Network Deployment Scenarios, Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1 #70bis, San Diego, Calif., USA, 8th-12th Oct. 2012.
A homogeneous network is a network of base stations (also referred to as NodeB's) in a planned layout providing communications services for a collection of user terminals (also referred to as user equipment nodes, UEs, and/or wireless terminals) in which all base stations may have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may offer unrestricted access to user terminals in the network, and each base station may serve roughly a same number of user terminals. Current cellular wireless communications systems in this category may include, for example, GSM (Global System for Mobile communication), WCDMA, HSDPA (High Speed Downlink Packet Access), LTE (Long Term Evolution), Wimax (Worldwide Interoperability for Microwave Access), etc.
In a heterogeneous network, low power node base stations (also referred to as low power nodes, LPNs, micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit nodes, RRU nodes, small cells, RRUs, etc.) may be deployed together with planned and/or regularly placed macro base stations, including macro base station MBS. Macro base station MBS may thus provide service over a relatively large macro cell area Mca, and each LPN may provide service for a respective relatively small LPN cell area Lca within the relatively large macro cell area Mca. Power transmitted by an LPN (e.g., 2 Watts) may be relatively small compared to power transmitted by a macro base station (e.g., 40 Watts for a typical macro base station). An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in macro cell area Mca of macro base station MBS and/or to off-load traffic from macro base station MBS (e.g., to increase capacity in a high traffic location, also referred to as a hot-spot). Due to the lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition(s).
The increasing use of wireless communication as well as the increased performance requirements thereof calls for improvements in various aspects, such as e.g. the handling of connections between the UEs and the base stations, in particular dual connectivity wherein the UEs have simultaneous connections to several base stations.